The goal of this project is to investigate the role that retinal Muller glial cells play in the maintenance of homeostasis in the retina. An understanding of the cellular physiology of Muller cells is a critical first step towards achieving that goal. It is proposed to address the regulation of intracellular free Ca2+ ion concentration [Ca2+]i in Muller cells since Ca2+ is an important and ubiquitous intracellular messenger. Muller cells are the predominant glial cells in the retina which span almost all retinal layers. These cells send out processes that surround almost all neuronal cell membrane in the retina and that closely approach synaptic profiles. Muller cells are, therefore, poised to "sense" the neuronal extracellular environment and are capable of responding to a variety of extracellular molecules, including neurotransmitters. The applicant has demonstrated that [Ca2+]i in Muller cells is increased in response to the excitatory retinal neurotransmitter, glutamate, as well as ATP and the neuroactive peptide, bradykinin. This project will involve the pharmacological characterization of plasma membrane receptors to these and other neurotransmitters that evoke Ca2+ responses in Muller cells. The transmitter-evoked [Ca2+]i increase appears first at the distal end of the Muller cell, the region that in vivo is associated with photoreceptor cell bodies and terminals, and then progresses slowly in a wave to the more proximal parts of the cell. The physiological basis of these Ca2+ waves will also be investigated as well as the intracellular transduction mechanisms that are involved in coupling receptor activation to increases in [Ca2+]i. Ca2+ imaging studies are proposed which are well-suited to this elongated cell since preliminary results indicate that there are consistent spatiotemporal patterns of changes in [Ca2+]i that can only be appreciated with videomicroscopy. An understanding of the cellular processes involved in neuron-to-Muller cell interactions and the intracellular Ca2+ signals that travel within Muller cells should lead to a better understanding of retinal function and homeostasis.